I believe regret is a wasted emotion. However it is human nature and we need to put things into perspective to overcome regret. I bet everyone wishes they had done more in the past.
I am 63 and can't ride as far as I used to. I was thinking that the other day while on a ride, then I told myself to just enjoy the fact that I can do as much as I do. In the past 6 years I have cycle toured in Europe 3 times and another 5 tours here in North America. How many people can or have done that? Sure, I wish I had toured more when I was younger, but do I have more tours in me? Yes, I just won't go as far each day. I know it's a cliche, but today is the first day of the rest of my life.